1. Field of the Invention
The present invention relates to an improvement of an internal meshing type planetary gear speed changing device in which an internal gear has teeth of circular arc profile, and an external gear has teeth of trochoidal profile inclusive of an epitrochoidal parallel curve, circular arc tooth profile or the like, so that rotation of an eccentric member fitted in the external gear causes the external gear to swingingly rotate, and that rotation thus input is output after being reduced in speed (or increased in speed) through the external and internal gears which are intermeshed with each other. More particularly, the invention relates to a structure of the internal meshing type planetary gear speed changing device which is made of plastic material such that it can be reduced in size and weight, and that it can be decreased in number of component parts, have a compact construction, and be produced at a low cost with high productivity.
2. Description of the Prior Art
There have been proposed various kinds of speed reducers in which internal meshing type planetary gear mechanisms are employed. One of these speed reducers is an internal meshing type planetary gear reducer well-known as a "Cyclo Speed Reducer" (trade mark) in which an internal gear has teeth of circular arc profile consisting of pins or combination of pins and rollers, and an external gear has teeth of trochoidal profile inclusive of an epitrochoidal parallel curve, with inner pins or inner pins and inner rollers being loosely fitted in the external gear, so that rotation of an eccentric member fitted in the external gear causes the external gear to swingingly rotate, and that rotation thus input is output after being reduced in speed through the external and internal gears which are intermeshed with each other. The "Cyclo Speed Reducer" is capable of transmitting a large torque, and its speed reduction ratio is large. Therefore, it is applied to various uses.
There is known another type of Cyclo Speed Reducer each component part of which is made of a plastic material for the purpose of further reducing the Cyclo Speed Reducer described above (hereinafter referred to as the internal meshing type planetary gear reducer) in size and weight and also in number of component parts. (See Japanese Utility Model Unexamined Publication No. 63-30648.)
This internal meshing type planetary gear reducer is made of plastic material which has not only flexibility but also a favorable sliding characteristic so that rotations are effected relatively smoothly and smooth transmission of torque is conducted in the reducer. Therefore, the internal meshing type planetary gear reducer made of plastic material is remarkably useful as a speed reducing rotational member in a domestic electric appliance, an office equipment, an automatic vending machine or the like.
One conventional example of the abovementioned internal meshing type planetary gear reducer made of plastic material will be described hereinafter with reference to FIGS. 11 and 12.
FIG. 11 is a cross-sectional view of the conventional example of internal meshing type planetary gear reducer, and FIG. 12 is a cross-sectional view of the same as FIG. 11, taken along the line XII--XII of FIG. 11.
In this example, rotation of an input shaft 1 is transmitted to an output shaft 2 in a state of being reduced in speed. The example may be, however, arranged in such a manner that the rotation reduced in speed is extracted from an internal gear 7, while the output shaft 2 being fixed.
An eccentric member 3 is integrally formed with the input shaft 1 made of a plastic material. An external gear 4 of a plastic material is fitted on the eccentric member 3. The external gear 4 is integrally provided with external teeth 5 of trochoidal profile on an outer periphery thereof. Inner pins 6 are integrally formed on the external gear 4. The inner pins 6 are formed to project from a side face of the external gear 4 in the direction substantially parallel to a longitudinal axis of the output shaft. The internal gear 7 of a plastic material also serves as a lateral wall casing on this side. Besides, this internal gear 7 is stationary in this example. The internal gear 7 is integrally formed with circular arc teeth 8 to mesh with the external teeth 5 of the external gear 4. The plastic output shaft 2 is integrally provided at the one end portion with a disk-like flange 9. This flange 9 includes inner-pin holes 10 into which the abovementioned inner pins 6 are inserted. The input shaft 1 is rotatably supported in a bearing hole 11 provided at the one end portion of the output shaft 2.
Operation of the internal meshing type planetary gear reducer having the above-described structure will now be explained.
The rotation of the input shaft 1 appears as swinging rotation of the external gear 4 via the eccentric member 3. Then, the swinging rotation of the external gear 4 is turned into rotation of the flange 9 reduced in speed through the inner pins 6, and the output force with low speed rotation of the flange 9 is transmitted to the output shaft 2.
The known internal meshing type planetary gear reducer made of plastic material as described has technical problems as follows.
FIG. 13 is a cross-sectional view showing a condition of contacts between the inner pins 6 and the inner-pin holes 10 at a moment of the operation.
Referring to FIG. 13, a center O.sub.1 of the external gear 4 provided with the inner pins 6 is eccentrically disposed by a distance e, apart from a rotational center O.sub.2 of the flange 9 (which is disposed at the same position as a rotational center of the output shaft 2) including the inner-pin holes 10. In the condition shown in the figure, the center O.sub.1 is just below the rotational center O.sub.2 at the distance e. Theoretically, the inner pin 6 at every location is in contact with the bottom of the associated inner-pin hole 10, as viewed in the figure.
A torque is transmitted when the inner pins 6 contact with the inner-pin holes 10. In the contact condition shown in FIG. 13, the torque transmission is mainly effected through the inner pins 6 and the inner-pin holes 10 located at positions X, Y, and Z because the inner pins 6 are swingingly rotated in a direction indicated b an arrow P in the figure. Contacts at the residual positions do not contribute to the torque transmission, and the resistive force interferes with the speed change function.
However, the conventional internal meshing type planetary gear reducer is so designed that the inner pins 6 at all the locations will be brought into contact with the respective inner-pin holes 10.
On the other hand, due to dimensional errors in producing individual parts, precise machining of such a gear mechanism that the inner pins 6 at all the locations are brought into contact with the inner-pin holes 10 will not be managed without difficulty or the cost for the machining will be unfavorably high. The reason is that many of internal meshing type planetary gear reducers made of plastic material, which are originally characterized by mass production at a low cost, are manufactured by injection molding which often results in molding distortion or contraction unsuitable for precise machining where very few production errors are allowed.
In the conventional internal meshing type planetary gear reducer made of plastic material, therefore, unnecessary contacts are induced owing to dimensional errors in producing individual parts so as to resist the speed change function, and the forcedly resistive sliding motion at contact portions is turned into internal load of the gear mechanism so as to increase friction resistance and noise, thus making it difficult for the gear mechanism to perform smooth transmission of the torque, with the life of the gear mechanism being shortened.
In order to solve these problems, as shown in FIG. 14, there have been proposed a method in which the diameter of each inner pin is made smaller than a theoretical value and a method in which the diameter of each inner-pin hole is made larger than a theoretical value. In these methods, however, backlashes due to play and looseness become unnecessarily larger, and controllability of normal/reverse rotation and accuracy of positioning are unfavorably deteriorated.